1. Field of the Invention
The present invention relates to a rare-earth alloy powder pressing apparatus and a rare-earth alloy powder pressing method, and more specifically to a rare-earth alloy powder pressing apparatus and a rare-earth alloy powder pressing method for example in which the rare-earth alloy powder fed in a cavity of a through hole of a die is pressed to form a compact.
2. Description of the Related Art
Referring to FIG. 12, a conventional rare-earth alloy powder pressing apparatus 1 comprises a frame 2 like a cage made of a magnetic material. Within the frame 2, a pair of pole pieces 4 for gathering a magnetic flux toward a cavity 3 is disposed. Each of the pole pieces 4 is surrounded by a coil 5 for generating a magnetic field, and a die 6 is disposed between the pole pieces 4. The die 6 includes a through hole 6a formed vertically, and auxiliary yokes 6b formed to flank the through hole 6a. The magnetic flux from the pole pieces 4 is concentrated by the auxiliary yokes 6b onto the cavity 3 for increased orientation of the alloy powder in the cavity 3. An upper punch 7a to be inserted into the through hole 6a from above is disposed above the die 6, whereas a lower punch 7b movable relatively to the die 6 is inserted in the through hole 6a. The cavity 3 formed by the through hole 6a and the lower punch 7b is supplied with the rare-earth alloy powder from a feeder box 8, and the rare-earth alloy powder in the cavity 3 is oriented by the magnetic field generated by electricity applied to the coils 5.
However, in manufacture of the rare-earth magnet by the rare-earth alloy powder pressing apparatus 1, if the powder is made by a quenching method, the compact strength is decreased since grain size distribution of the powder concentrates in a narrow range. If the powder is added with a lubricant for improved compression and orientation, then the compact strength is decreased. As a result, the obtained compact is soft and brittle, having a low compact density of 3.9 g/cm3xcx9c4.6 g/cm3 for example. Therefore, when the compact is taken out of the die 6, the compact collapses if rubbed against walls of the through hole 6a over a long distance. To avoid this, the alloy powder is pressed at a shallow depth from an upper surface of the die 6.
Further, the auxiliary yokes 6b are disposed near the cavity 3 for increased orientation at the time of pressing. However, generally, the auxiliary yokes 6b are formed not to project out of the upper surface of the die 6 since the feeder box 8 slides on the upper surface of the die 6. Therefore, as shown in FIG. 13, a magnetic flux A becomes asymmetric in a vertical direction of the cavity 3 due to leakage of the magnetic flux above the cavity 3. Further, the orienting magnetic field becomes weaker in an upper portion of the cavity 3 than in the other portion thereof, having a weaker magnetic flux density. Thus, the alloy powder in the cavity 3 is drawn more strongly to the other portion, and as a result, green density after the pressing operation becomes lower in the upper portion of the cavity 3. If this compact is sintered, the sintered body 9 often becomes bent as shown in FIG. 14, due to uneven shrinkage at the time of sintering. This problem is especially serious in a product which is narrow and long in the direction of the orientation since difference in shrinkage after the sintering is significantly larger, resulting in an unacceptable bending and therefore decreased yield of the product.
It is therefore a primary object of the present invention to provide a rare-earth alloy powder pressing apparatus and a rare-earth alloy powder pressing method capable of reducing the bending in the obtained sintered body.
According to an aspect of the present invention, there is provided a rare-earth alloy powder pressing apparatus for manufacture of a compact by pressing a rare-earth alloy powder fed in a cavity of a through hole of a die, comprising: a first punch and a second punch for pressing the rare-earth alloy powder fed in the cavity; and a correcting yoke disposed near the die at a time of orientation, on a side from which the compact is taken out, for correction of a magnetic field in the cavity closer to a symmetry in a direction vertical to a direction of an orienting magnetic field.
According to another aspect of the present invention, there is provided a rare-earth alloy powder pressing method for manufacture of a compact by pressing a rare-earth alloy powder fed in a cavity of a through hole of a die, the method comprising: a first step of feeding the rare-earth alloy powder in the cavity; and a second step of orienting the rare-earth alloy powder, with a correcting yoke disposed near the die, on a side from which the compact is taken out.
According to another aspect of the present invention, a rare-earth magnet obtained by sintering a compact manufactured by the above method is provided.
According to the present invention, at the time of orientation, the correcting yoke is disposed near the die, on the side from which the compact is taken out. Therefore, when the cavity is fed with the rare-earth alloy powder and the orienting magnetic field is applied, intensity distribution of the magnetic field in the cavity becomes generally symmetric in the direction vertical to the direction in which the orienting magnetic field is applied. As a result, the green density distribution of the compact becomes generally symmetric in the direction vertical to the direction in which the orienting magnetic field is applied, making possible to reduce the bending of the sintered body. Therefore, the yield of the product can be improved.
Preferably, the correcting yoke is disposed on an upper side of the die if the compact is to be taken out from the upper side of the die, whereas the correcting yoke is disposed on a lower side of the die if the compact is to be taken out from the lower side of the die.
Further, preferably, the die includes an auxiliary yoke: The correcting yoke and the auxiliary yoke respectively have inward side surfaces, and the correcting yoke and the auxiliary yoke are disposed at the time of orientation, with the inward side surfaces respectively made flush with a plane vertical to the direction of the orienting magnetic field. In this case, since the inward side surfaces of the correcting yoke and the auxiliary yoke are respectively made flush at the time of orientation, it becomes easier to symmetrize the magnetic field.
Further, preferably, the correcting yoke is disposed near the die only at the time of orientation. Therefore, the correcting yoke does not interfere with movement of the feeder box and a cleaner.
Preferably, a plurality of the correcting yokes are prepared, and the apparatus further comprises a connecting member for connection of the plurality of correcting yokes with each other. In this case, since the correcting yokes can be fixed with each other by the connecting member, the correcting yokes are rigidly held in position even if a strong magnetic field is generated.
Further, preferably, the correcting yoke is urged toward the die at the time of orientation. Therefore, the correcting yoke can be reliably fitted onto the surface of the die, making more possible to symmetrize the magnetic field in the cavity.
Further, preferably, the correcting yoke and one of the first and second punches disposed on the side where the correcting yoke is disposed are supported by a same supporting member. In this case, for example, the two members can be moved vertically by a single means, leading to a simpler constitution.
Preferably, the correcting yoke and the die are formed integrally with each other. In this case, the constitution of the apparatus can be simplified.
The above objects, other objects, characteristics, aspects and advantages of the present invention will become clearer from the following description of embodiments to be presented with reference to the accompanying drawings.